EVIDENCE FOR CONIFER-DERIVED LEAF WAX N-ALKANES IN PALEOGENE SEDIMENTS AND IMPLICATIONS FOR THEIR HYDROGEN AND CARBON ISOTOPES

Long chain n-alkanes are leaf wax biomarkers used as paleoclimate proxies because their carbon (δ¹³C) and hydrogen (δD) isotopes are sensitive to biological and environmental processes. Both angiosperms and conifers produce n-alkanes, but these two major plant groups have unique δ¹³C and δD distributions due to physiological differences, such as water use efficiency strategies. Previous work on extant species and early Cenozoic paleoflora sites in the Bighorn Basin, Wyoming suggest that North American angiosperms produce significantly more (up to 200x) n-alkanes than conifers. Therefore, conifers do not significantly contribute n-alkanes to the sediment record. However, this assumption may be problematic where conifers were locally abundant on the landscape in the geologic past. To examine the paleovegetation source of sediment n-alkanes and their isotopic signatures, we collected a suite of sediment samples from several Paleocene and Eocene fossil leaf sites in Ellesmere and Axel Heiberg Islands in the High Arctic and from the Oligocene Creede Formation site in Colorado, where conifers were locally abundant. From each sediment sample, we extracted, quantified, and measured the δ¹³C values of terpenoids (triterpenoids and diterpenoids), an independent class of plant biomarkers. Whereas triterpenoids are produced exclusively by angiosperms, diterpenoids are produced exclusively by conifers. In addition, the δ¹³C composition of triterpenoids and diterpenoids are distinct to each plant group. After accounting for biosynthetic fractionation, which occurs when different compound classes are synthesized from carbon fixed during photosynthesis, the δ¹³C values of n-alkanes from these sites more closely match the δ¹³C values of diterpenoids than that of the triterpenoids, implying a conifer-derived vegetation source for n-alkanes. These results provide evidence that conifers have the potential to affect the isotopic composition of sediment n-alkanes. Thus, understanding the paleovegetation source of leaf wax n-alkanes may be especially important during times of rapid climate change where δD and δ¹³C values preserved in the sediment can reflect a complex signal of rapidly changing flora communities and climate.